I'd summarize Web MIDI implementation status on Chrome. I also did it at the end of the last year, but in Japanese. So, I'd mention also previous status together.

Chrome 30

chrome://flags#enablr-web-midi appeared on OS X

Input ports were available on OS X, but there was a bug on sysex until Chrome 32

Chrome 31

Output ports were also available on OS X

Infobar and settings UI were implemented as a part of content settings

Chrome 32

IAC driver was available

Chrome 33

Web MIDI worked on Windows, but had wrong timestamp values

Chrome 34

Web MIDI worked on Linux (but only ALSA supporting systems)

Also, worked on Android, but only output ports are available

There was a problem on initialization process, so the first API call didn't work on Android

Real-time messages were available from this version

System messages were available without a sysex permission from this version (previous versions required a sysex permission even for system messages)

Error provided detailed messages on invalid send messages

Infobar appeared even on local files, but browser could not remember the permission because of security reason

Wrong timestamp was fixed on Windows

Chrome 35

Now, Promise is not limited version Promise, MIDIAccessPromise, but fully featured proper Promise

Now the API is available on Chrome OS

Input ports are available on Android, but the initialization issue is not fixed yet

Fix a bug that sending messages had wrong CoreMIDI timestamp and caused a problem that DAW could not record Chrome sending messages with correct timestamp

MIDIAccess.sysexEnabled attribute is added

Chrome 36

Android initialization issue will be fixed

Chrome 37 (planned)

Linux and Chrome OS supports will be improved so to support ALSA sequencer API

Infobar and settings UI for Android will be implemented

Note: The implemented API is slightly different from the latest W3C spec. You can see the old specification here. Also, MIDIConnectionEvent is not implemented yet. It will be available once new API is ready.